Knowing more ways of tackling more problems makes you a better programmer. The more difference there is between what you do at home and what you do at work, the more you will learn. If at work you write C/C++/Java, learn Haskell at home. If you do GUIs at work, do low level coding at home. (And vice versa, of course.) Me, I play with SMT solvers at work and I build my own keyboards at home.